Process of and apparatus for air separation with nitrogen quenched power turbine

ABSTRACT

Air is separated by low temperature rectification using a 150400 psia. column and a 45-140 psia. column, with nitrogen-rich gas from the latter quenching hot combustion gas prior to work expansion of the resulting intermediate temperature gas mixture.

I United States Patent 1 m1 3,731,495 Coveney 1 May 8, 1973 [541 PROCESS OF AND APPARATUS FOR 2,955,434 [/1960 Cost ..62/l8 AIR SEPARATION WITH NITROGEN 3,397,548 8/1968 Ergcnc t t t 4.62/29 2,499,043 2/1950 Voorhees r ..62/39 QUENCHED POWER TURBINE 3,070,966 1/1963 Ruhemann ..62/39 [75] Inventor: Joseph Francis Coveney, North 3,446,014 5/1969 Foster-Pegg.... .....60/39.l8 B Tonawanda, N.Y. 3,466,884 9/1969 Ruckborn i t ..62/38 2,520,862 8/1950 Swearingen 1.62/29 [73] Assignee: Union Carbide Corporation, New

York Primary Examiner-Norman Yudkoff [22] Filed; 28, 1970 Assistant Examiner-Arthur F. Purcell Atl0rney--Paul A. Rose, Harrie M. Humphreys, John [2]] Appl- N05 101,992 C. Le Fever and Lawrence G. Kastriner 52 US. Cl. ..62/39, 62/13, 62/18, 1 ABSTRACT 62/29 60/3913 B, 4 Air is separated by low temperature rectification using [5]] Int. Cl. ..F25] 3/04, F253 3/00 3 50400 psia column and a 45 40 i column, [58] Field of Search ..62/18, 27, 28, 29, with nitrogemflm gas f the latter quenching hot 60/39, 18 B, 39, 23 combustion gas prior to work expansion of the resulting intermediate temperature gas mixture. 1 [56] References Cited 22 Claims, 5 Drawing Figures UNITED STATES PATENTS 3,280,555 /1966 Charpentier ..60/39.23

A g; T TNEEFQDETYOJDRESLETW 7 q I I P- J41 5 33 v VI 1 6'3 I g ad, V 62 FUEL W i576 V 6i C l 1 76' 56 l i 76 come. 76 5 3&{1 F 67 l 1 g 34d 1 5a 55, 121 77 l Y 6:; 4 V 2 51 I 39 v 2a I 43 1 7g 340 V 6): '56, 60 I as /9 i 11/ 35 I 39 47 I HELPER g7 FEED I 66' r kw] 57 x i STEAM COMP. EXPANDER AIR I I 2 l 1 TURBINE 0MP. I i 35 A I 5 i v- I L a Q L I WASTE B.F.W. 4 585g? l V 4 1 i 274 25 5 1 64 T EXPORT i l I 1 AIR STEAM EXHAUST GAS 45 i Pmm-ed May a, 1973 5 Shoots-Sheet 1 PmOQxu QNL N INVENTOR JOSEPH F. COVENEY ATTORNEY Patented May 8, 1973 3,731,495

5 Shoots-Sheet 2 6'5 jg L INVENTOR JOSEPH F. COVENEY ATTORNEY A k L Patented Maya, 1973 3,131,495

5 Shoots-Sheet 5 INVENTOR ATTORNEY Patented May 8, 1973 5 Sheets-Sheet 4.

INVENTOR JOSEPH F.. COVENEY' azm ATTORNEY Patented May 8, 1973 3,731,495

5 Shoots-Sheet 5 INVENTOR JOSEPH F. COVENEY 304 6.

ATTORNEY PROCESS or AND APPARATUS FOR AIR SEPARATION WITH NITROGEN QUENCHED POWER TURBINE BACKGROUND OF THE INVENTION tion employs a higher pressure column having its upper end in heat exchange relation with the lower end of a lower pressure stage rectification column. Cold compressed air is separated into oxygen-enriched and nitrogen-rich liquids in the higher pressure column and these liquids are transferred to the lower pressure column for separation into nitrogen and oxygen products. One of the limitations of this system is that the lower pressure column usually operates at slightly above atmospheric pressure, e.g., psia., so that the products discharged from this column must usually be compressed prior to storage and/or ultimate consumption. This requires a very expensive clean product compression system which is in addition to the separate compression system for the feed air. Another disadvantage is that the pressure ratio of the higher to the lower pressure column (and in the heat exchangers processing fluids from these columns) is quite high, as for example 3.6 (based on a temperature difference of 3K), when the higher pressure column operates at 90 psia. This means that considerable power is lost in the system in the inevitable throttling processes between higher and lower pressure columns. Still another disadvantage of these prior art systems is the relatively large equipment, e.g., the rectification column, piping and heat exchanger passageways required to process the 25 psia. fluids.

An object of this invention is to provide an improved method of and apparatus for air separation of the two pressure level (double column) rectification type, ,which requires less power and smaller, less expensive compression equipment than prior art systems.

Other objects and advantages of this invention will be apparent from the ensuing disclosure and appende claims.

SUMMARY This invention relates to a process of and apparatus for low temperature rectification of air into oxygen and nitrogen. One embodiment of the process aspect of this invention relates to air separation by low temperature rectification wherein cold compressed air is separated into oxygen-enriched and nitrogenrich liquids in a higher pressure rectification column having its upper end in heat exchange relation with the lower end of a lower pressure stage rectification column with the liquids being transferred to the lower pressure rectification column for separation into nitrogen and oxygen products. This process is characteri'zed by the steps of compressing the feed air to no more than 140 psia., dividing the compressed air into a major part and a minor part, and mixing the minor part as oxidant with fuel for burning in a combustion zone to form relatively hot combustion gas. Relatively cool nitrogen gas is injected into this hot combustion gas in a separate quenching zone to form an intermediate temperature nitrogen-enriched gas mixture at super-atmospheric pressure, and the latter is expanded to lower pressure with the production of external work. Part of this external work is recovered as the energy for the aforementioned feed air compression. Another part of this external work is used for further compressing the major part of the compressed air to at least 150 psia. and sufficient to operate the higher pressure rectification column within a pressure range of 150-400 psia.

The major part of the compressed air is partially cooled either before or after the further compression. Atmospheric impurities are removed from the major part also before or after the further compression step, and the cleaned air is further cooled whereupon at least a major part of the further cooled air is introduced to the higher pressure rectification column. The lower pressure rectification column is operated at -140 psia. and sufficient for discharging nitrogen-rich gas therefrom, heat exchanging at least the major part thereof with said cleaned further compressed air for partially warming said nitrogen-rich gas and providing part of the further cooling of the cleaned further compressed air, heat exchanging the partially warmed nitrogen-rich gas with the major part of the compressed air for further warming thereof and said partially cooling of the air, and employing the further warmed nitrogen-rich gas as the relatively cool nitrogen gas to be injected into the hot combustion gas.

Oxygen product gas is discharged from the lower pressure rectification column and heat exchanged with the cleaned further compressed air as another part of the required refrigeration for further cooling this air prior to its low temperature rectification.

One embodiment of the apparatus aspect of this invention is characterized by a base compressor for compressing feed air to first super-atmospheric pressure no more than l4O psia., a combustion chamber and means for introducing fuel thereto, conduit means for flowing a minor part of the compressed feed air from the base compressor to the combustion chamber, a quenching chamber separate from. the combustion chamber and r means for passing hot combustion gas from the latter to the former. Conduit means are also provided for inject- 7 feed air from the base compressor thereto for further compression to second super-atmospheric pressure of at least psia. Rotatable shaft means join the base compressor, the booster compressor and the turbine expander for transferring the shaft work from the ex pander to each of these compressors as the required energy for driving same. i

Means are provided for removing atmospheric impurities from the major part of compressed feed air.

Second heat exchanger means are used for further cooling the cleaned further compressed air.

A double rectification column comprises a higher pressure stage for operation at 150-400 psia., a lower pressure stage for operation at 45-140 psia. and a heat exchanger joining the upper end of the higher pressure stage and the lower end of the lower pressure stage. Separate conduit means are employed for transferring oxygen-enriched liquid from the higher pressure stage lower end to the lower pressure stage, and transferring nitrogen-rich liquid from the upper end of the higher pressure stage to the lower pressure stage.

Conduit means are included for flowing at least a major part of the further cooled cleaned further compressed air from the second heat exchanger means to the higher pressure stage of the double rectification column for separation therein. Other conduit means discharge nitrogen-rich gas from the lower pressure rectification stage and flow same to the second heat exchanger means for heat exchange therein with the cleaned further compressed air as part of said further cooling. Still other conduit means are used for flowing the partially warmed nitrogen-rich gas from the second heat exchanger means to the first heat exchanger means for heat exchange therein with the major part of compressed feed air for said cooling thereof. Finally, conduit means pass further warmed nitrogen-rich gas from the first heat exchanger means into the hot combustion gas as said relatively cool nitrogen-rich gas in the quenching chamber.

Conduit means are provided for discharging oxygen product gas from the lower pressure stage of the double rectification column and flowing same to the second heat exchanger for heat exchange therein with the cleaned further compressed air as another part of the further cooling thereof.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic flowsheet of a complete system for air separation into both oxygen and nitrogen gas products and employing a single expansion turbine for product nitrogen according to one embodiment of the invention;

FIG. 2 is a schematic flowsheet of an alternative low temperature air separation section C for marginal liquid product production using two product nitrogen expansion turbines;

FIG. 3 is a schematic flowsheet of another alternative low temperature air separation section C characterized by a single nitrogen-rich liquid transfer to the lower pressure rectification nitrogen gas;

FIG. 4 is a schematic flowsheet of still another alternative low temperature air separation section .C characterized by work expansion of partially cooled air and by passing same around the rectification column, i.e., excess air; and

FIG. 5 is a further alternative section C characterized by work expanding the major part of further cooled ai.

DESCRIPTION OF PREFERRED EMBODIMENTS Referring now to the drawings and in particular FIG. 1, the system comprises three main sections: power section A, pre-cleanup section B and low temperature air separation section C. Feed air is introduced through conduit and pressurized in first base compressor 11 to first super-atmospheric pressure as for example 97 psia. and discharged in conduit 12. It is thereafter divided into two parts, a minor part in branch conduit 13 and a major part in conduit 14. The minor part, for example Comprising percent by volume of the total feed air, is directed to combustion chamber 15 along with fuel introduced through conduit 16. This fuel may comprise any clean burning combustible fluid material as for example oil or a gas mixture including a combustible such as methane or carbon monoxide. Sufficient air is introduced through conduit 13 to combustion chamber 15 to ensure complete oxidation of the fuel and typically 20-30 percent stoichiometric excess of air is used for this purpose. The mixture is burned in chamber 15 to form a relatively hot combustion gas at superatmospheric pressure and for example 3,000F. This hot gas is passed through conduit 16 to quenching chamber 17 for mixing therein with relatively cool nitrogen-rich gas injected therein through conduit 18. These gases are mixed in quenching chamber 17 to form an intermediate temperature gas mixture for example at 1,600F., also at super-atmospheric pressure as for example 97 psia. The nitrogen-enriched gas mixture is passed through conduit 16 to turbine expander 19 for production of external work and discharge therefrom at lower pressure, e.g. l5 psia. and lower temperature, e.g. 930F.

Combustion chamber 15 and quenching chamber 17 are separate in the sense that they are at least baffle-divided. For example, the combustion chamber 15 may be located on the frame of the compressor-turbine assembly in the form of multiple cylindrical chambers clustered around the compressor and discharging directly into the turbine blading. It is normally of all metal construction and by appropriate blanketing of the metal with nitrogen-rich quench gas, the nitrogenenriched gas temperature entering expander 19 may be as high as 1,650F. Alternatively, combustion chamber 15 quenching chamber 17 may be located separate from the compressor'turbine assembly frame, with the compressed air transported from compressor 11 to combustion chamber 15 quenching chamber 17 and thence to turbine 19 through pipes or ducts. The offframe quenching chamber 17 may be lined with refractory insulation so that the highest attainable intermediate temperature is lower than with metal walls and on the order of l',350F. In commercially available embodirnents, both the combustion zone and the quenching zone are within the same compartment or chamber and are separated only by a baffle. If the two zones 15 and 17 are contained within separate vessels connected by conduit 16 as illustrated, the latter must withstand the combustion temperature of 3,000F.

The major part of the compressed air in conduit 14 is partially cooled in passageway 14a of first heat exchanger 14b to still above-ambient temperature for removal of its heat of compression and then further compressed to a second super-atmospheric pressure as for example 335 psia. in booster air compressor. 20. The energy required to drive base compressor 11 and booster compressor 20 is provided by the shaft work from expander 19 with connection 21. The latter may include a speed-changing device such as a gear box. For example, two-thirds of the external work from expander 19 may be used to operate base compressor 11 and the remaining one-third employed as energy to drive booster air compressor 20. Alternatively, expander 19 may be oversized in terms of the air compression energy requirements for low temperature air separation section C, so that more external work is produced than is needed for air compression energy. In this event, the remainder of the turbine output, available as shaft work, may be employed by other energyconsuming means, e.g. electric generators. Excess air discharged from compressor 11 may be diverted from conduit 12 to conduit 16 downstream combustion chamber by conduit 12a and control valve 12b therein, and is not included in the air divided into major and minor parts by conduits l4 and 13, respectively. It should also be understood that although shaft coupling of turbine expander 19 with base compressor ll and booster compressor 20 is required in the apparatus of this invention, the external work may be indirectly transferred to one or both of these compressors according to the methd of the invention. For example, the expander 19 may be directly coupled to an electric generator and the latter may in turn be electrically joined to driving means for the compressors.

The heat contained in the expanded gas discharged from turbine 19 into conduit 22 is preferably recovered, as for example by heating water introduced through conduit 23 to boiler 24. The cooled expanded gas is exhausted from boiler 24 through discharge conduit 25 and the steam, preferably at high pressure, elg. 600 psia. and 750F., is discharged from boiler 24 into conduit'26. All or a portion of the steam may be exported through branch conduit 27. Alternatively or additionally, steam may be work expanded in helper turbine 28. The latter is preferably coupled into the power train through common shaft 21 to base compressor 11, turbine 19 and booster compressor 20. The low pressure expanded steam discharged from helper turbine 28 into conduit 29 at perhaps 420F. may for example be injected into quenching chamber 17 along with the nitrogen gas mixture, as illustrated in FIG. 1. In this manner the quantity of gas required for this purpose from low temperature air separation section C is minimized. Alternatively, the low pressure expanded steam in conduit 29 may be condensed and recirculated to boiler 24. In either instance, the work produced by helper turbine 28 is utilized. If desired, an auxiliary air compressor (not shown) may be joined to the power train through shaft 21 to absorb a portion of the work generated by the train, asfor example the work Output of helper turbine 28.

The further compressed air discharged from booster compressor 20 at for example 335 psia. into conduit 30 may be chilled for moisture condensation by conventional means not illustrated, and passed to pre-cleanup section B for flow-through either of adsorption traps 31 and 32 for selective adsorption of air impurities, i.e. B 0, C0 and C l-l The adsorbent may for example be the molecular sieve known commercially as calcium zeolite A. Adsorption traps 31 and 32 are piped in parallel flow relation with appropriate valving and interconnections at the inlet and discharge ends for the flow sequencing to insure continuous operation. After loading with air impurities, the adsorption traps are cleaned as for example by the sequence of partial depressurization, heating for desorption with nitrogenrich gas separated in section C but ultimately to be wasted after use as quench gas, cooling with the same waste nitrogen gas, and repressurization by feed air from conduit 30. As illustrated, the further compressed air is flowed through inlet valve 33 to adsorption trap 31 for cleaning therein, and discharged through outlet".

valve 34a. During this period, nitrogen-rich gas from air separation section C is diverted by blower 34b in conduit 34c, warmed in heater 34d for example to 550F. and directed into adsorption trap 32 through its air outlet end for desorption and removal of the remaining impurities not desorbed during the partial depressurization step. The atmospheric impurity-containing purge gas is discharged from the air inlet end of trap 32 and returned through conduit 34e to the main stream of nitrogen-rich gas. During the cooling step, the flow is the same except that the nitrogen-rich gas is not warmed in heater 34d and preferably enters trap 32 at about ambient temperature.

The cleaned further compressed air is next directed to second heat exchanger 35 of air separation section C forfurther cooling therein to a lower below-ambient temperature which is preferably at least 10K above saturation at the existing pressure, eg about l34K at 320 psia. In particular, the air is introduced to passageway 36 therein, and is cooled by two separate streams of nitrogen-rich gas and oxygen product gas.

The larger nitrogen-rich gasstream is introduced to passageway 37 and will hereinafter be referred to as the nitrogen waste gas. The smaller nitrogen-rich gas stream is introduced to passageway 38 and will hereinafter be referred to as the nitrogen product gas.

The oxygen product gas is introduced to second heat exchanger 35 in passageway 39.

The further cooled air entering higher pressure column 40 is preferably superheated at least 10K above saturation. This superheat flashes downwardly flowing oxygen-enriched liquid and thereby causes additional upward vapor flow, improving rectification ef ficiency. The prior art has usually cooled the feed air to substantially saturation temperature as this was deemed necessary to remove the air impurities by deep cooling for near-complete deposition in reversing flowtype heat exchangers. In this preferred embodiment, the air impurities are removed by selective adsorption prior to the second further cooling step, as previously described.

The cold air is separated intoioxygen-enriched liquid" which accumulates in the column lower end, and.

nitrogen-rich gas at the upper end. The liquid, which may for example comprise about 36 percent oxygen, is withdrawn through transfer conduit 41 and preferably divided into two parts for subcooling prior to introduction in lower pressure rectification column 42. One part is directed to third heat exchanger 43 and passageway 44 therein for subcooling against colder work expanded product nitrogen. in passageway 45. The other part of the oxygen-enrichedrliquid is directed to fourth heat exchanger 46 and-passageway 47 therein oxygen-enriched liquid at the lower end, e.g. 12 percent O may be withdrawn therebetween and through transfer conduit 51. This liquid may then be subcooled by fifth heat exchanger 52 in passageway 53 by the aforementioned colder product nitrogen gas in passageway 54 and waste nitrogen gas in passageway 55. The subcooled liquid is throttled through valve 56 and fed into lower pressure rectification column 42 at an intermediate level for separation therein and as reflux for the column.

The nitrogen-rich vapor reaching the upper end of higher pressure column 40 may for example contain perhaps ppm. oxygen. This vapor is directed through conduit 57 to heat exchanger 58 for liquefaction by the oxygen-rich liquid from the lower pressure column which in turn is at least partially vaporized. The nitrogen is at the higher pressure of column 40 whereas the oxygen is at the lower pressure of column 42. The pressure ratio of the nitrogen to oxygen must be sufficient to provide the temperature difference required to obtain the heat exchange, e.g. l3KAT.

Due to the higher pressures characteristic of both rectification stages of this process, the pressure ratio is lower than normally practiced in double rectification columns for air separation, i.e. less than 3.4 (based on a 3KAT). As previously indicated, typical operating pressures for such columns at the nitrogen-rich vapor and oxygen-rich liquid heat exchange are and 90 psia., so that the pressure ratio is on the order of 3.6. The pressure across second heat exchanger is also lower than corresponding prior art heat exchangers, so that the gases separated in column 42 and flowing to this exchanger may not have sufficient volume to remove all of the air impurities in passageways by flow reversing heat exchange relationships with the compressed feed air. Anotherproblem is that self-cleaning requires the cold end AT to be closed in reversing heat exchangers, but at the high pressures characteristic of thisinvention the difference in specific heats of the streams tends to spread the cold end AT widely. A further consideration is that reversing heat exchangers do not remove all the impurities from the air and cold end gel traps are needed'forfinal purification before the air enters the rectification column. At the'high pressures of this invention, the temperature at the cold end of second heat exchanger 35 is considerably higher than in low pressure plants and substantially more impurity would escape freeze out in a reversing heat exchanger. This means that the burden placed upon final purification would be greater and the cold end gel traps would be prohibitively large. Accordingly, second heat exchanger 35 is not the flow reversing type and the atmospheric air impurities must be removed prior to flowing the further compressed major part to the low temperature air separation section C. Otherwise these impurities will be deposited in the low temperature heat exchangers and the rectification columns. This clean-up is accomplished in the previously described air clean-up section B.

The nitrogen-rich liquid formed in heat exchanger 58 is divided into two portions. One part is returned 42 upper end into conduit 63 as for example 105 psia and may be 99.999 percent pure. This cold stream may be flowed through passageway 54 of fifth heat exchanger 52 for subcooling of the transfer liquid, from higher pressure column 40 and thereby partially warmed or superheated. It may then be further warmed in passageway 48 of fourth heat exchanger 46 by subcooling oxygen-enriched liquid in passageway 47. The

further warmed nitrogen product at about 1 l9K is now preferably work expanded in turbine 64 to lower pressure as for example 56 psia. and thereby recooled to about 104K. The recooled product nitrogen subcools oxygen-enriched liquid by flowing through passageway 45 of third heat exchanger 43 and then is directed to passageway 38 for recovery of its remaining sensible refrigeration by the further compressed air in passageway 36. If required by the end use, this product nitrogen gas emerging from the warm end of passageway 38 may be recompressed by means not illustrated.

The product oxygen gas is discharged from the lower end of lower pressure column 42 into conduit 65 at for example 111 psia. and ll5K and is directed through passageway 39 of second heat exchanger 35 where its sensible refrigeration is also recovered by the further compressed air in passageway 36. The warmed oxygen I product gas may also be recompressed if desired.

The waste nitrogen gas is discharged from an intermediate level of lower pressure column 42.into conduit 66 and may for example comprise 92% N 8% 0 This cold gas is partially warmed in passageway 55 of fifth heat exchanger 52 against the transfer liquids from lower pressure column 40, then further warmed in passageway'49 of fourth heat'exchanger 46 by subcooling oxygen-enriched transfer liquid in passageway 47.

The further warmed waste nitrogen at about I l9K still contains considerable sensible refrigeration below am-' bient temperature, and this is recovered in second heat exchanger 35 by flow through passageway 37 in heat exchange relation with the further compressed cooling air in passageway 36.

As previously described in connection with the preclean-up section B, a portion of the waste nitrogen discharged from second heat exchanger 35 (and broadly described herein as nitrogen-rich gas) is diverted from conduit 66 by blower 34b as the purging--- cooling gas for adsorption traps 31 and 32, and returned to conduit 66 by conduit 342.

The nitrogen-richer waste gas discharged from the preclean-up and air separation sections B and C in conduit 66 is directed to the passageway 67 .in first heat exchanger 14b where it partially cools the major part of compressed air in passageway 14a. and removes the heat of compression, e.g. to 265F. Although the nitrogen-rich waste gas discharged from first heat exchanger 14b is above ambient temperatures, e.g. about 470F., it is cool relative to the 3,000F. combustion gas discharged from chamber 15 into conduit 16, and is injected therein through conduit 18 for quenching in the previously described manner.

The aforedescribed invention provides important advantages over prior art low temperature air separation systems. The oxygen and nitrogen gas products are delivered under pressure, and the cost and complexities of product compression are therefore greatly reduced or in some instances eliminated. Since the cost of product compression is often a large item in an air separation plant, the reduction of this cost by the invention has a significant effect on the overall cost of the product. Based on the FIG. 1 embodiment, for larger plants this cost reduction for 98% N and 98% O is about 5-10 percent assuming N and gas products are supplied at 50 and 100 psia. respectively. Another advantage of the invention is lower plant investment, by virtue of thesmaller equipment required to process the higher pressure fluids. Again based on a large plant, the FIG. 1 embodiment permits a saving on the order of percent in the initial investment.

There are numerous modifications to the FIG. 1 embodiment which have been contemplated as part of the invention. For example, the first heat exchanger 14b for partially cooling the major part of compressed air and further warming the nitrogen-rich gas may be located in booster compressor discharge conduit 30 instead of inlet conduit 14. Also, preclean-up section B for removing air impurities may be located in conduit 14 upstream of booster compressor 20.

As another modification, the air impurities could be removed in non-reversing flow type heat exchangers wherein the impurities are deposited in passageways cooled by cold gas from the system or externally supplied refrigerant. For impurity deposition it is necessary tolcool the feed. air to the dew point of the impurity at the existing pressure, as is well understood by those skilled in the art. For continuous operation it is necessary to provide duplicate non-reversing flow type heat exchangers so that a passageway loaded with impurities may be cleaned, and. a previously cleaned passageway may be placed in the air flow path. The impurity-loaded passageway may for example be cleaned by flowing a heated purge gas therethrough in a manner analogous to the regenerating procedure previously described in connection with adsorbent traps 31 and 32.

FIGS. 2-5 illustrate certain modifications of the FIG. 1 low temperature air separation section C which could be employed with the FIG. 1 power section A and precleanup section B. Only the features different from the previously described air separation section C will be discussed, and items which correspond to FIG. 1 items are identified by the same numeral. The numerals at the end of certain conduits identify the component in pre-cleanup section B to which the conduit is joined.

FIG. 2 illustrates an embodiment adapted for liquid oxygen production in addition to gaseous oxygen and nitrogen production. Product nitrogen gas discharged from column 42 upper end into conduit 63 is partially superheated to for example 1 l7K in passageway 54 of fifth heat exchanger 52 and subcools the transfer liquids from higher pressure column 40 in passageways 53 and 61. It is then work expanded in first turbine 70 to slightly above saturation, as for example l0lK and 58 psia. Next it is resuperheated in heat exchanger 71 and subcools a third part of the higher purity nitrogen liquid in passageway 72, the latter having been diverted from conduit through conduit 73. The resuperheated product nitrogen is further work expanded in second turbine 74 before entering passageway 48 of fourth heat exchanger 46 for subcooling part of the oxygen-enriched liquid in passageway 47. Additional refrigeration may be recovered from this further work expanded product nitrogen by flow through passageway .75 in heat exchanger 76 for subcooling a fourth part of the nitrogen-rich liquid diverted from conduit 60 to conduit 77 and thence to passageway 75. Liquid oxygen product is withdrawn from the base of lower pressure column 42 through conduit 79 having control valve 80 therein.

In the FIG. 3 embodiment, the high purity nitrogen separation section at the upper end of the lower pressure column 42 is eliminated',-and only one nitrogenrich liquid is transferred to this column through conduit 60. Nitrogen gas in conduit 63 is consecutively superheated in fifth heat exchanger 52 and fourth heat exchanger 46, then divided into two parts. One part in conduit 81 is directed to passageway 37 of second heat exchanger 35 for further cooling of the cleaned further compressed air in passageway 36. Another part of the superheated nitrogen gas is directed through branch conduit 82 to turbine 83 for work expansion, then resuperheated in passageway 45 of third heat exchanger 43 by subcooling oxygen-enriched liquid in passageway 44. This resuperheated gas is directed to passageway 84 of second heat exchanger 35 where its remaining sensible refrigeration is recovered by the cleaned further compressed air in passageway 36, and then wasted.

In FIG. 4, a portion of the cleaned further compressed air is diverted by conduit 85 from passageway 36 of second heat exchanger 35 at an intermediate temperature level. This diverted portion is expanded in turbine 86 to the pressure of lower pressure column 42 and thereafter partially rewarmed in passageway 45 'by subcooling oxygen-enriched liquid. in passageway 44 of third heat exchanger 43. This diverted work expanded partially rewarmed air is further rewarmed in passageway 87 of heat exchanger 88 by additionally cooling a part of the further compressed and further cooled air diverted from conduit 30 to branch conduit .89 and passageway 90. The additionally cooled air'is then introduced to higher pressure column 40 for separation. The further rewarmed expanded air emerg ing from heat exchanger 88 in conduit 85 joins the nitrogen gas in conduit 63 to form the waste gas in conduit 66. In this embodiment; the compressed air expanded for refrigeration is not separated in the rectifiupon the refrigeration requirements, for example upon whether liquid products are to be withdrawn and if so, whether the quantity of liquid products is large or small. This is in contrast to the previously described embodiments wherein the discharge pressure of compressor 20 need onlyexceed the operating level of the higher pressure rectification stage by an amount sufficient to overcome friction losses in the connecting conduits.

More particularly in FIG. the feed air is compressed in machine 20 to a pressure between about 200 and 700 psia., precleaned in section B and passed through conduit 30 to heat exchanger 35 for further cooling. The first larger portion of this further cooled air is flowed through conduit 91 to work expander 93 where its pressure is reduced to the level of the higher pressure rectification stage 40, i.e., to within the range 150 to 400 psia. Refrigeration is thereby produced and the temperature of the expanded air drops further to near-saturation corresponding to the discharge pressure. The second smaller portion of the further cooled air is diverted through conduit 92 to passageway 95 of heat exchanger 94. It is thus further cooled and liquefied by gaseous nitrogen product flowing from conduit 81 to passageway 96. The liquefied second smaller portion is then throttled in valve 97 to the operating pressure of the higher pressure rectification stage 40 and is admitted to the latter column for separation.

Heat exchanger 94 contributes significantly to the efficiency of the FIG. 5 embodiment. Liquefying a portion of the feed air at elevated pressure stabilizes the temperature of the cold nitrogen entering heat exchanger at a relatively warm level. This in turn stabilizes the temperature of further cooled air in conduit 91 entering the work expander 93 at about the same level. Thus, work expander 93 can be operated to produce maximum refrigeration.

Additional steps for producing still more refrigeration can be combined with the FIG. 5 embodiment, as for example the work expansion steps illustrated in FIGS.'l-4. However, the embodiment of FIG. 5 alone is capable of producing liquid product such as liquid oxygen withdrawn from the base of lower pressure rectification stage 40 through conduit 98, provided that the expansion ratio across turbine 93 is sufficient to provide the needed refrigeration.

What is claimed is:

1. In a process for air separation by low temperature rectification wherein cold compressed air is separated into oxygen-enriched liquid and nitrogen-rich liquid in a higher pressure rectification column having its upper end in heat exchange relation with the lower end of a lower pressure stage rectification column with said liquids being transferred to said lower pressure rectification column for separation into nitrogen and oxygen products, the improvement comprising: compressing feed air to no more than 140 psia.; dividing the compressed air into a major part and a minor part; mixing said minor part as oxidant with fuel for burning in a combustion zone to form relatively hot combustion gas; injecting relatively cool nitrogen-rich gas into said hot combustion gas in a separate quenching zone to form an intermediate temperature nitrogen-enriched gas mixture at super-atmospheric pressure; expanding the intermediate temperature gas mixture to lower pressure with the production of external worki recovering part of said external work as the energy for said compressing of feed air; partially cooling the major part of compressed air; further compressing said major part of compressed air to at least psia. and sufficient to operate said higher pressure stage rectification column using another part of the external work from said expanding as the energy required for said further compressing; removing atmospheric impurities from said major part of compressed air; further cooling the cleaned further compressed air and introducing at least the major part thereof to said higher pressure rectification column; operating the lower pressure rectification column at 45-140 psia. and sufficient for discharging nitrogen-rich gas therefrom, heat exchanging at least the major part thereof with said cleaned further compressed air for partially warming said nitrogen-rich gas and providing part of said further cooling of the cleaned further compressed air, heat exchanging the partially warmed nitrogen-rich gas with the major part of the compressed air for said partial cooling thereof and further warming of said nitrogen-rich gas, and employing the further warmed nitrogen-rich gas as said relatively cool nitrogen gas to be injected into said hot combustion gas; and discharging oxygen product gas from said lower pressure rectification column and heat exchanging same with said cleaned further compressed air as another part of the required refrigeration for said further cooling.

2. A low temperature air separation process according to claim 1 wherein nitrogen product gas is discharged from the upper end of said lower pressure rectification column as a minor part of said nitrogenrich gas, work expanded to lower pressure, and heat exchanged with said cleaned further compressed air as still another part of the refrigeration for said further cooling.

3. A low temperature air separation process according to claim 2 wherein the work expanded nitrogen product gas is heat exchanged with said oxygen-enriched liquid from the lower end of said higher pressure rectification column for subcoo'ling said oxygen-enriched liquid, prior to said heat exchanging Withsaid cleaned further compressed air for said further cooling.

4. A low temperature air separation process according to claim 1 wherein all of the further cooled and- 6. A low temperature air separation process accord- I ing to claim 1 wherein the atmospheric impurities are removed from the further compressed air by selective adsorption thereof, the adsorbent is periodically regenerated by diverting a minor part of the partially warmed nitrogen-rich gas, heating same, passing the heated minor part through said adsorbent for purging thereof; and the atmospheric impurity-containing purge gas is returned to the undiverted partially warmed nitrogen-rich gas for said heat exchanging with said major part of the compressed air.

7. A low temperature air separation process according to claim 1 wherein the work expanded gas mixture is heat exchanged with water to generate high pressure steam, said high pressure steam is expanded to lower pressure so as to produce external work, and said external work is recovered as part of the energy required for the feed air compression and further compression.

8. A low temperature air separation process accord ing to claim 1 wherein all of said quenching of said heat combustion gas is provided by said relatively cool nitrogen-rich gas.

9. A low temperature air separation process according to claim 7 wherein the work expanded steam is injected into said quenching zone for cooling of said hot combustion gas along with said nitrogen-rich gas.

10. A low temperature air separation process according to claim 2 wherein said nitrogen product gas is work expanded to a first lower pressure, heat exchanged with a warmer liquid withdrawn from said higher pressure rectification column for subcooling thereof prior to introduction of such liquid to said lower pressure rectification column, thereafter work expanded from said first lower pressure to a second lower pressure and then heat exchanged with another warmer liquid withdrawn from said higher pressure rectification column prior to the heat exchanging with said cleaned further compressed air, and wherein a product liquid is discharged from said lower pressure rectification column.

11. A low temperature air separation process according to. claim 1 wherein said further cooling of said cleaned further compressed air is to temperature at least 10K above the saturation point thereof at the existing pressure.- 1

12. A process for air separation by low temperature rectification comprising the steps of: i

a. compressing feed air to first super-atmospheric pressure no more than 140 psia. using part of the external work from succeeding expansion (f) as the'energy for said compressing;

b. dividing the compressed feed air into a major part and a minor part, and passing said minor partto a combustion zone as oxidant;

c. introducing fuel to said combustion zone and burning same with said oxidant to form relatively hot combustion gas at super-atmospheric pressure;

d. passing said relatively hot combustion gas to a quenching zone separate from said combustion zone;

e. injecting relatively cool nitrogen-rich gas into said relatively hot combustion gas in said quenching zone so as to mix same and form an intermediate temperature nitrogen-enriched gas mixture at super-atmospheric pressure; I

f. expanding the intermediate temperature nitrogenenriched gas mixture to lower pressure so as to produce external work;

g. further compressing the major part of said compressed feed air from said first super-atmospheric pressure to second super-atmospheric pressure of 150-400 psia., and using another part of said external work as the energy for said further compression;

h. partially cooling the major part of compressed air;

i. removing atmospheric impurities from said major part of the compressed air;

j. further cooling the cleaned further compressed air;

k. rectifying at least the major part of the further compressed cold air in a higher pressure rectification column so as to produce oxygen-enriched liquid at the lower end and nitrogen-rich gas at pressure of -400 psia. at the upper end of said column;

l. heat exchanging said nitrogen-rich gas with oxygen-rich liquid at pressure of45-l40 psia. so as to condense said nitrogen-rich gas as reflux for said higher pressure rectification column and a lower pressure rectification column while simultaneously vaporizing said oxygen-rich liquid as vapor for upward flow through said lower pressure rectification column;

m. throttling part of the nitrogen-rich liquid from heat exchange (1) and said oxygen-enriched liquid fro to higher pressure rectification column to 45-140 psia. and introducing each liquid into said lower pressure rectification column for rectification against the oxygen-rich vapor to produce said oxygen-rich liquid at the column lower end and nitrogen-rich gas in the column upper region;

n. discharging said nitrogen-rich gas from said lower pressure rectification column and heat exchanging at least the major part thereof with said cleaned further compressed air to partially warm said nitrogen-rich gas and to provide part of the refrigeration for further cooling (j);

0. heat exchanging the partially warmed nitrogenrich gas with said major part of the compressed feed air as said partial cooling (h); and thereby further warming said nitrogen-rich gas;

p. employing the further warmed nitrogen-rich gas from heat exchanging (o) as said relatively cool nitrogen-rich gas for the injecting and mixing (e); and

q. discharging oxygen product gas from said lower pressure rectification column and'heatexchanging same with said cleaned further compressed air as.

an additional part of the refrigeration for further o g (0- 13. Apparatus for air separation by low temperature rectification comprising:

a. a base compressor for compressing feed air to first super-atmospheric pressure no more than [40' psia.;

b. a combustion chamber and means for introducing fuel thereto; 0. conduit means for flowing a minor part of the compressed feed air from compressor (a) to comg. first heat exchanger means for partially coolingsaid major part of compressed feed air;

h. a booster compressor and conduit means for passing the major part of compressed feed air from base compressor (a) thereto for further compression thereof to second super-atmospheric pressure of at least 150 psia;

. rotatable shaft means joining base compressor (a), booster compressor (h) and turbine expander (f) for'transferring the shaft work from expander (f) to each of compressors (a) and (h) as the required energy for driving same;

j. means for removing atmospheric impurities from said major part of compressed feed air;

. second heat exchanger means for further cooling cleaned further compressed air;

I. a double rectification column comprising a higher pressure stage for operation at 140-400 psia., a

lower pressure stage for operation at 45-140 psia.,

and a heat exchanger joining the upper end of the higher pressure stage and the lower end of the lower pressure stage, separate conduit means for transferring oxygen-enriched liquid from the higher pressure stage lower end to the lower pressure stage and transferring nitrogen-rich liquid from the upper end of the higher pressure stage to the lower pressure stage;

m. conduit means for flowing at least a major part of the further cooled cleaned further compressed air from second heat exchanger means (k) to the higher pressure stage of column (I) for separation therein;

. conduit means for discharging nitrogen-rich gas from the lower pressure rectification stage of column (1) and flowing same to second heat exchanger means (k) for heat exchange therein with said cleaned further compressed air as part of said further cooling;

. conduit means for flowing the partially warmed nitrogen-rich gas from second heat exchanger means (k) to first heat exchanger means (g) for heat exchange therein with said major part of compressed feed air for said partial cooling thereof;

conduit means for passing further warmed nitrogen-rich gas from first heat exchanger means g) to conduit injectionmeans (e) as said relatively cool nitrogen-rich gas;

. conduit means for discharging oxygen product gas from said lower pressure stage of column (I) at 45-140 psia'. and flowing same to second heat exchanger means (k) for heat exchange therein with said cleaned further compressed air as another part of said further cooling.

14. Apparatus for air separation according to claim 13 including a second turbine for expanding nitrogen product gas discharged from the upper end of the lower pressure stage of double column (I) to lower pressure so as to produce external work; and conduit means for passing the work expanded nitrogen product gas to second heat exchanger means (k) for heat exchange therein with said cleaned further compressed air as still another part of said further cooling.

15. Apparatus for air separation according to claim 14 including third heat exchanger means joined to the liquid transfr conduit means of (l) for subcooling at least part of said oxygen-enriched liquid from said higher pressure stage lower end prior to introduction in said lower pressure stage of column (I), and also joined to said conduit means for work expanded nitrogen product gas for partial warming of such gas prior to said passing to second heat exchanger means (k).

16. Apparatus for air separation according to claim 14 including fourth heat exchanger means joined to the liquid transfer conduit means of (l) for subcooling at least part of said oxygen-enriched liquid from; said higher pressure stage lower end prior to introduction in said lower pressure stage of column (I); and wherein first conduit means joined to the upper end of said lower pressure stage of column (1) for flowing said nitrogen product gas to and discharging partially warmed nitrogen product gas from said fourth heat exchanger, and second conduit means joined to an intermediate level of said lower pressure stage for flowing nitrogen waste gas to and discharging partially warmed nitrogen waste gas from said fourth heat exchanger means comprise conduit means (n).

17. Apparatus for air separation according to claim 14 including second liquid conduit means for discharging oxygen-enriched liquid from an intermediatelevel of said higher pressure stage of column (I) and transferring same to an intermediate level of said lower pressure stage; fifth heat exchanger means joined to said second liquid conduit means and said conduit means of (l) for transferring nitrogen-rich liquid, for subcooling each of said liquids prior to introduction in said lower pressure stage; and wherein first gas conduit means joined to the upper end of said lower pressure stage of column (1) for flowing said nitrogen product gas to and discharging partially warmed nitrogen product gas from said fifth heat exchanger means and to said second turbine, and second gas conduit means joined to an intermediate level of said lower pressure stage for flowing nitrogen waste gas to and-discharging partially warmed nitrogen waste gas from said fifth heat exchanger means comprise conduit means (m).

18. Apparatus for air separation according to claim 13 wherein impurity removal means (j) comprises a multiplicity of selective adsorbent traps; diverting conduit means joined to conduit means (0) at one end and the cleaned air discharge end of said adsorbent traps at the other end for flowing partially warmed nitrogenrich gas thereto; gas blower means in said diverting conduit means; gas heater means in said diverting conduit means; return conduit means joined at one end to conduit means (0) between said diverting conduit means and said first exchanger means (g), and joined to the impurity-containing feed air inlet end of said adsorbent traps at the other end; and control valve means joined to the air inlet and discharge ends of said adsorbent traps arranged to-flow feed air through one trap for impurity adsorption and simultaneously regenerate at least one other trap previously loaded with impuri-' ties by flow of heated nitrogen-rich purge gas therethrough, and thereafter cycle the feed air and nitrogen-rich purge gas flows between the traps.

19. Apparatus according to claim 13 including a waste heat boiler, conduit means for passing the work expanded nitrogen-enriched gas mixture to said boiler;

water supply means to said boiler for heat exchange with the gas mixture to generate high pressure steam, a steam turbine for expanding said high pressure steam to lower pressure so as to produce external work, and rotatable coupling means joining said steam turbine and rotatable shaft means (i).

20. Apparatus according to claim 19 including conduit means for flowing the expanded lower pressure steam to said quenching chamber (d) for cooling of said hot combustion gas.

21. A low temperature air separation process according to claim 1 wherein a first larger portion of the,

; said heat exchanging with said cleaned further compressed air, the so-liquefied second smaller portion is throttled to the pressure of said higher pressure rectification column, and the work expanded first larger portion and throttled liquefied second smaller portion of air are each introduced to said higher pressure stage rectification column.

22. Apparatus for air separation according to claim 13 including a turbine in conduit means (m) for expanding a first larger portion of said further cooled cleaned further compressed air to the pressure of the higher pressure rectification stage, additional heat exchanger means for still further cooling and liquefying a second smaller portion of said further cooled cleaned further compressed air being arranged and constructed in flow communication with conduit means (n) such that said nitrogen-rich gas is introduced thereto as the refrigerant and passed therefrom to second heat exchanger means (k), means for throttling the liquefied second smaller portion to the pressure of the higher pressure rectification stage, and conduit means for introducing the so-throttled liquid to said higher pressure rectification stage. 

2. A low temperature air separation process according to claim 1 wherein nitrogen product gas is discharged from the upper end of said lower pressure rectification column as a minor part of said nitrogen-rich gas, work expanded to lower pressure, and heat exchanged with said cleaned further compressed air as still another part of the refrigeration for said further cooling.
 3. A low temperature air separation process according to claim 2 wherein the work expanded nitrogen product gas is heat exchanged with said oxygen-enriched liquid from the lower end of said higher pressure rectification column for subcooling said oxygen-enriched liquid, prior to said heat exchanging with said cleaned further compressed air for said further cooling.
 4. A low temperature air separation process according to claim 1 wherein all of the further cooled and further compressed cold air is introduced to said higher pressure rectification column.
 5. A low temperature air separation process according to claim 1 wherein the absolute pressure ratio of said nitrogen-rich vapor from the upper end of said higher pressure rectification column and oxygen-rich liquid from the lower end of said lower pressure rectification column in said heat exchange relation is less than 3.4 based on a 3*K Delta T.
 6. A low temperature air separation process according to claim 1 wherein the atmospheric impurities are removed from the further compressed air by selective adsorption thereof, the adsorbent is periodically regenerated by diverting a minor part of the partially warmed nitrogen-rich gas, heating same, passing the heated minor part through said adsorbent for purging thereof; and the atmospheric impurity-containing purge gas is returned to the undiverted partially warmed nitrogen-rich gas for said heat exchanging with said major part of the compressed air.
 7. A low temperature air separation process according to claim 1 wherein the work expanded gas mixture is heat exchanged with water to generate high pressure steam, said high pressure steam is expanded to lower pressure so as to produce external work, and said external work is recovered as part of the energy required for the feed air compression and further compression.
 8. A low temperature air separation process according to claim 1 wherein all of said quenching of said heat combustion gas is provided by said relatively cool nitrogen-rich gas.
 9. A low temperature air separation process according to claim 7 wherein the work expanded steam is injected into said quenching zone for cooling of said hot combustion gas along with said nitrogen-rich gas.
 10. A low temperature air separation process according to claim 2 wherein said nitrogen product gas is work expanded to a first lower pressure, heat exchanged with a warmer liquid withdrawn from said higher pressure rectification column for subcooling thereof prior to introduction of such liquid to said lower pressure rectification column, thereafter work expanded from said first lower pressure to a second lower pressure and then heat exchanged with another warmer liquid withdrawn from said higher pressure rectification column prior to the heat exchanging with said cleaned further compressed air, and wherein a product liquid is discharged from said lower pressure rectification column.
 11. A low temperature air separation process according to claim 1 wherein said further cooling of said cleaned further compressed air is to temperature at least 10*K above the saturation point thereof at the existing pressure.
 12. A process for air separation by low temperature rectification comprising the steps of: a. compressing feed air to first super-atmospheric pressure no more than 140 psia. using part of the external work from succeeding expansion (f) as the energy for said compressing; b. dividing the compressed feed air into a major part and a minor part, and passing said minor part to a combustion zone as oxidant; c. introducing fuel to said combustion zone and burning same with said oxidant to form relatively hot combustion gas at super-atmospheric pressure; d. passing said relatively hot combustion gas to a quenching zone separate from said combustion zone; e. injecting relatively cool nitrogen-rich gas into said relatively hot combustion gas in said quenching zone so as to mix same and form an intermediate temperature nitrogen-enriched gas mixture at super-atmospheric pressure; f. expanding the intermediate temperature nitrogen-enriched gas mixture to lower pressure so as to produce external work; g. further compressing the major part of said compressed feed air from said first super-atmospheric pressure to second super-atmospheric pressure of 150-400 psia., and using another part of said external work as the energy for said further compression; h. partially cooling the major part of compressed air; i. removing atmospheric impurities from said major part of the compressed air; j. further cooling the cleaned further compressed air; k. rectifying at least the major part of the further compressed cold air in a higher pressure rectification column so as to produce oxygen-enriched liquid at the lower end and nitrogen-rich gas at pressure of 150-400 psia. at the upper end of said column; l. heat exchanging said nitrogen-rich gas with oxygen-rich liquid at pressure of 45-140 psia. so as to condense said nitrogen-rich gas as reflux for said higher pressure rectification column and a lower pressure rectification column while simultaneously vaporizing said oxygen-rich liquid as vapor for upward flow through said lower pressure rectification column; m. throttling part of the nitrogen-rich liquid from heat exchange (1) and said oxygen-enriched liquid from the higher pressure rectification column to 45-140 psia. and introducing each liquid into said lower pressure rectification column for rectification against the oxygen-rich vapor to produce said oxygen-rich liquid at the column lower end and nitrogen-rich gas in the column upper region; n. discharging said nitrogen-rich gas from said lower pressure rectification column and heat exchanging at least the major part thereof with said cleaned further compressed air to partially warm said nitrogen-rich gas and to provide part of the refrigeration for further cooling (j); o. heat exchanging the partially warmed nitrogen-rich gas with said major part of the compressed feed air as said partial cooling (h); and thereby further warming said nitrogen-rich gas; p. employing the further warmed nitrogen-rich gas from heat exchanging (o) as said relatively cool nitrogen-rich gas for the injecting and mixing (e); and q. discharging oxygen product gas from said lower pressure rectification column and heat exchanging same with said cleaned further compressed air as an additional part of the refrigeration for further cooling (j).
 13. Apparatus for air separation by low temperature rectification comprising: a. a base compressor for compressing feed air to first super-atmospheric pressure no more than 140 psia.; b. a combustion chamber and means for introducing fuel thereto; c. conduit means for flowing a minor part of the compressed feed air from compressor (a) to combustion chamber (b); d. a quenching chamber separate from combustion chamber (b) and means for passing hot combustion gas from chamber (b) thereto; e. conduit means for injecting relatively cool nitrogen-rich gas into said hot combustion gas in quenching chamber (d) to form an intermediate temperature nitrogen-enriched gas mixture; f. a turbine for expanding said intermediate temperature nitrogen-enriched gas mixture to lower pressure so as to produce external work; g. first heat exchanger means for partially cooling said major part of compressed feed air; h. a booster compressor and conduit means for passing the major part of compressed feed air from base compressor (a) thereto for further compression thereof to second super-atmospheric pressure of at least 150 psia; i. rotatable shaft means joining base compressor (a), booster compressor (h) and turbine expander (f) for transferring the shaft work from expander (f) to each of compressors (a) and (h) as the required energy for driving same; j. means for removing atmospheric impurities from said major part of compressed feed air; k. second heat exchanger means for further cooling cleaned further compressed air; l. a double rectification column comprising a higher pressure stage for operation at 140-400 psia., a lower pressure stage for operation at 45-140 psia., and a heat exchanger joining the upper end of the higher pressure stage and the lower end of the lower pressure stage, separate conduit means for transferring oxygen-enriched liquid from the higher pressure stage lower end to the lower pressure stage and transferring nitrogen-rich liquid from the upper end of the higher pressure stage to the lower pressure stage; m. conduit means for flowing at least a major part of the further cooled cleaned further compressed air from second heat exchanger means (k) to the higher pressure stage of column (l) for separation therein; n. conduit means for discharging nitrogen-rich gas from the lower pressure rectification stage of column (l) and flowing same to second heat exchanger means (k) for heat exchange therein with said cleaned further compressed air as part of said further cooling; o. conduit means for flowing the partially warmed nitrogen-rich gas from second heat exchanger means (k) to first heat exchanger means (g) for heat exchange therein with said major part of compressed feed air for said partial cooling thereof; p. conduit means for passing further warmed nitrogen-rich gas from first heat exchanger means (g) to conduit injection means (e) as said relatively cool nitrogen-rich gas; q. conduit means for discharging oxygen product gas from said lower pressure stage of column (l) at 45-140 psia. and flowing same to second heat exchanger means (k) for heat exchange therein with said cleaned further compressed air as another part of said further cooling.
 14. Apparatus for air separation according to claim 13 including a second turbine for expanding nitrogen product gas discharged from the upper end of the lower pressure stage of double column (l) to lower pressure so as to produce external work; and conduit means for passing the work expanded nitrogen product gas to second heat exchanger means (k) for heat exchange therein with said cleaned further compressed air as still another part of said further cooling.
 15. Apparatus for air separation according to claim 14 including third heat exchanger means joined to the liquid transfer conduit means of (l) for subcooling at least part of said oxygen-enriched liquid from said higher pressure stage lower end prior to introduction in said lower pressure stage of column (l), and also joined to said conduit means for work expanded nitrogen product gas for partial warming of such gas prior to said passing to second heat exchanger means (k).
 16. Apparatus for air separation according to claim 14 including fourth heat exchanger means joined to the liquid transfer conduit means of (l) for subcooling at least part of said oxygen-enriched liquid from said higher pressure stage lower end prior to introduction in said lower pressure stage of column (l); and wherein first conduit means joined to the upper end of said lower pressure stage of column (l) for flowing said nitrogen product gas to and discharging partially warmed nitrogen product gas from said fourth heat exchanger, and second conduit means joined to an intermediate level of said lower pressure stage for flowing nitrogen waste gas to and discharging partially warmed nitrogen waste gas from said fourth heat exchanger means comprise conduit means (n).
 17. Apparatus for air separation according to claim 14 including second liquid conduit means for discharging oxygen-enriched liquid from an intermediate level of said higher pressure stage of column (l) and transferring same to an intermediate level of said lower pressure stage; fifth heat exchanger means joined to said second liquid conduit means and said conduit means of (l) for transferring nitrogen-rich liquid, for subcooling each of said liquids prior to introduction in said lower pressure stage; and wherein first gas conduit means joined to the upper end of said lower pressure stage of column (l) for flowing said nitrogen product gas to and discharging partially warmed nitrogen product gas from said fifth heat exchanger means and to said second turbine, and second gas conduit means joined to an intermediate level of said lower pressure stage for flowing nitrogen waste gas to and discharging partially warmed nitrogen waste gas from said fifth heat exchanger means comprise conduit means (m).
 18. Apparatus for air separation according to claim 13 wherein impurity removal means (j) comprises a multiplicity of selective adsorbent traps; diverting conduit means joined to conduit means (o) at one end and the cleaned air discharge end of said adsorbent traps at the other end for flowing partially warmed nitrogen-rich gas thereto; gas blower means in said diverting conduit means; gas heater means in said diverting conduit means; return conduit means joined at one end to conduit means (o) between said diverting conduit means and said first exchanger means (g), and joined to the impurity-containing feed air inlet end of said adsorbent traps at the other end; and control valve means joined to the air inlet and discharge ends of said adsorbent traps arranged to flow feed air through one trap for impurity adsorption and simultaneously regenerate at least one other trap previously loaded with impurities by flow of heated nitrogen-rich purge gas therethrough, and thereafter cycle the feed air and nitrogen-rich purge gas flows between the traps.
 19. Apparatus according to claim 13 including a waste heat boiler, conduit means for passing the work expanded nitrogen-enriched gas mixture to said boiler; water supply means to said boiler for heat exchange with the gas mixture to generate high pressure steam, a steam turbine for expanding said high pressure steam to lower pressure so as to produce external work, and rotatable coupling means joining said steam turbine and rotatable shaft means (i).
 20. Apparatus according to claim 19 including conduit means for flowing the expanded lower pressure steam to said quenching chamber (d) for cooling of said hot combustion gas.
 21. A low temperature air separation process according to claim 1 wherein a first larger portion of the further cooled further compressed air is work expanded to the pressure of said higher pressure column, a second smaller portion of said further cooled further compressed air is still further cooled and liquefied by heat exchange with said nitrogen-rich gas discharged from said lower pressure rectification column prior to said heat exchanging with said cleaned further compressed air, the so-liquefied second smaller portion is throttled to the pressure of said higher pressure rectification column, and the work expanded first larger portion and throttled liquefied second smaller portion of air are each introduced to said higher pressure stage rectification column.
 22. Apparatus for air separation according to claim 13 including a turbine in conduit means (m) for expanding a first larger portion of said further cooled cleaned further compressed air to the pressure of the higher pressure rectification stage, additional heat exchanger means for still further cooling and liquefying a second smaller portion of said further cooled cleaned further compressed air being arranged and constructed in flow communication with conduit means (n) such that said nitrogen-rich gas is introduced thereto as the refrigerant and passed therefrom to second heat exchanger means (k), means for throttling the liquefied second smaller portion to the pressure of the higher pressure rectification stage, and conduit means for introducing the so-throttled liquid to said higher pressure rectification stage. 